A technical article published in the year 2000 entitled “Developmental trends in cheese production” (H.-P. Bachmann, H. Schaer, R. Sieber, H. Winkler, F. Rentsch) of the Federal Institute for Dairy Farming, Switzerland, “New technologies in cheese production” is described under Section 3. This article emphasizes that in the new technology for cheese production the treatment of the milk is of prime importance. On the one hand it is a matter of making the milk as free of bacteria as possible and on the other hand the chemical composition of the milk is to be changed in a purposeful manner.
In processes for sterilization of milk, in addition to the various known heat treatment processes (thermization, pasteurization) microfiltration using separation membranes is presented, the pore diameter of the membrane of 1.4 μm all additional skim milk enables components to pass through the membrane and only the bacteria and spores to be retained. Since the bacteria and the fat globules have approximately the same size, in membrane filtration the fat must be centrifuged off before microfiltration and only the skim milk is microfiltered. The resulting retained material (bacteria concentrate) and the cream are flash pasteurized with an ultraflash pasteurizing temperature system at 130° C. for 4 seconds and then returned again to the process milk. In this way milk which is largely free of spores can be processed. It has been shown using a pilot system with a capacity of 500 l milk per hour that the concentration factor is 20:1 (5% retained material) and that the butyric acid bacteria and spores during microfiltration by means of separation membranes are separated with very high efficiency, the number of other bacteria also being reduced.
In addition to the aforementioned membrane separation processes the aforementioned technical article also discloses sterilization separation, so-called bactofugation being the best known process for reducing bacteria. In the pertinent process which has been known for more than 20 years, use is made of the effect that the spores are specifically heavier than the milk. In order to improve efficiency, the sterilization separation is often carried out twice in succession. Other sterilization processes are so-called high pressure processes in which the treatment of the milk is done with hydrostatic pressure from 1,000 to 10,000 bar for two to 60 minutes at 20 to 30° C. In these processes the vegetative microorganisms in particular are effectively killed by the pressure on their membrane. Spores and enzymes are however largely resistant. Moreover the casein micelles and whey proteins are adversely changed.
Furthermore, ultrafiltration is known as another membrane separation process in which using a pressure difference high-molecular substances are separated from low-molecular substances and concentrated. In ultrafiltration of skim milk the proteins (caseins, whey proteins) of the milk are retained by the membrane in the retained material, while salts, lactose, and low-molecular nitrogen compounds of the membrane pass into the permeated substance. With an increasing degree of concentration the dry mass and protein content in the UF retained material rise as a consequence, the ratio of casein to whey protein however remains unchanged. The pertinent ultrafiltration membrane separation process in a pretreatment stage of the milk however allows an increase of the incorporation of native whey proteins; this benefits the quality of the milk.
The above described sterilization processes are expensive to use in terms of process engineering. If the processes use separation membranes, the high price of the membrane contributes to the expense. The milk fat (lipids) generally is to be removed before the actual separation process, in order to prevent clogging of the membrane. The clogging of the filter is also called blocking. Commercial use with large amounts of aqueous dispersions, such as milk for sterile filtration, therefore cannot be considered.
In order to improve the process steps on a commercial scale and in particular to ensure fully automated operation, microfiltration systems with ceramic membranes sold under the trademark TETRA ALCROSS™ MB by the Tetra Pak company are used. They likewise resort to microfiltration by means of separation membranes, which however, are made in this case as ceramic membranes of aluminum oxide and/or other metal oxides, with a pore size between 0.1 to 1.4 μm. As a result of using ceramic membranes they can be sterilized with hot water, can be highly loaded, are chemically resistant in a wide pH range, and have high bursting pressures. The criteria for selection of ceramic membranes are formed by the retention rate for protein and the desired effects of protein fractionation, for example, separation of casein and whey proteins and the separation of microorganisms. The ceramic membranes used produce a low, constant and uniform transmembrane pressure along the entire membrane surface, which allows a high and constant throughput. With the pertinent microfiltration systems a generic process may be carried out, fully automatic operation being attainable with high throughputs. As a result of using ceramic membranes as the separation membranes however the known process is also more expensive and complex in implementation. Furthermore the ceramic membranes likewise tend to block.
PCT-WO 96/32021 discloses a generic process which however allows only bacteria reduction of milk serum, i.e., only of the continuous phase of a dispersion. Therefore the disclosed process according to the PCT publication relates to filtration of a homogenous aqueous solution, charged (polar) deep-bed filters with their filter media being used as the deep-bed filter means. With the pertinent known process using charged deep-bed filter media, the sterilization of two phase systems of the type of an aqueous liquid-liquid dispersion as a two-phase mixture such as milk would not be possible and in this connection, as suggested by the PCT publication, should filter aids be used, their immediate blocking and thus their becoming unusable would have to be expected.
A similar process is also the subject matter of EP-A-0 798 003 which allows titer reduction of viruses in an aqueous solution, likewise using electrically charged deep-bed filter media. With the respective approach case the bacteria reduction of a nonhomogeneous two-phase or multiphase mixture, such as milk or skim milk, likewise is not possible. If in the disclosed approach polar (charged) filter media were to be used in two-phase mixtures, in static filtration, immediate blocking of the filter medium would occur.